The magnitude of subsurface contamination in the United States today is such that it poses a serious threat, both in terms of the health of humans and wildlife near it, and with regard to the environment as well. The state of the technology available to clean-up subsurface contamination sites has often lagged far behind environmental regulations; and when such technology has proven effective, it has also usually been too expensive to employ.
Subsurface contamination may occur in a variety of soil formations. The expression "soil formation" as used herein is intended to mean any geologic formation comprising soil, weathered rock, or sound rock, or any combination of these. Soil comprises unconsolidated mineral grains, while sound rock, i.e., bedrock, comprises consolidated or lithified aggregations of mineral grains. Weathered rock is sound rock in the process of becoming soil through the eroding activity of the elements upon it. The methods and apparatus of the present invention are applicable to all such soil formations, including bedrock. Although bedrock may contain natural fractures and voids, the present invention can be used to significantly increase the number and extent of such fractures and voids.
The subsurface soils comprising the contamination sites to be remediated by present invention are generally divided into two major zones, (1) the unsaturated zone, also known as the "vadose zone" and (2) the saturated zone. Perched water zones are also included. The present invention is fully applicable to, and operable in, both of these zones, and is especially suitable for non-cohesive soils such as granular sands and gravels, which do not fracture, in the conventional meaning of the term. The vadose zone extends from the ground surface down to the ground water table. The saturated zone begins at the ground water table and extends to a further depth. The vadose zone may be further divided into additional subzones, but will be treated as a single zone in the present discussion. Since the vadose zone is the uppermost layer of the terrestrial environment, it is more likely to contain pathways for toxic and hazardous chemicals to enter groundwater systems.
Studies have shown that it is less costly to remove volatile organic compounds (VOCs) from the vadose zone than to pump and treat contaminated ground water. That is the reason for the current focus on technology for the in situ removal of VOCs from the vadose zone. Such treatment technologies include vapor extraction, biodegradation, soil washing and thermal treatment. Vapor extraction is a process for the in situ removal of VOCs by mechanically extracting soil gases from the vadose zone through one or more vertically oriented perforated vent wells installed in the contaminated zone. Air is forced to travel through the pore space in the soil, causing volatilization of the liquid and adsorbed volatile organic compounds. The extracted soil gases are then either vented to the atmosphere or into an emission control system, depending on the concentration. The two major embodiments of such vapor extraction processes which have been demonstrated successfully in field use are in situ air stripping and vacuum extraction. In situ air stripping employs a series of interconnected air injector vents which are supplied with forced air by an above ground blower and manifold system that forces the air into the soil through the perforated vent wells. A separate blower or vacuum pump and manifold system is used to apply negative pressure to air extraction vents to withdraw the soil gases. The injection and extraction vents are located alternately within the array of vent wells on the site. This approach functions best with highly permeable soils, e.g., loose, sandy soils and has proven to be much less effective in tightly packed soils and in soils with a high clay content.
Biodegradation is another process which has been used effectively in the treatment of soils contaminated with organic compounds. In biodegradation, or bioremediation, the ecological conditions in the soil are altered to enhance microbial catabolism or to cometabolise the organic contaminant, thus transforming it into a simpler, non-toxic product. Indigenous microorganisms are utilized most often, although seeding of the soil with exogenous microorganisms is also possible. Microorganisms are either aerobic, anaerobic, or facultative anaerobic, which can grow either in the absence or presence of oxygen. The most effective treatment has been the aerobic microbial process. With this process, oxygen and often nutrients are injected or infiltrated into the subsurface environment, using wells or a percolation process. The major factors which affect the rate of biodegradation in the vadose zone include: pH, temperature, water content, carbon content, clay content, oxygen, nutrients, the nature of the microbial population, acclimation and concentration. However, unfavorable reaction kinetics, low substrate concentration and slow degradability of certain compounds remain significant problems.
Another major limiting factor in bioremediation has been the low permeability of the fine-grained soil layers present at many sites. On the other hand, a site contaminated with methylene chloride, n-butyl alcohol, acetone and dimethylaniline, after three years of in situ aerobic biological treatment, has had its contaminant plume reduced by 90%. Reclamation of an aquifer contaminated with benzene, toluene, and xylene using biodegradation has been achieved, with emphasis on the importance of oxygenating the subsurface environment, and in particular obtaining superior rates of biodegradation using hydrogen peroxide as an oxygen donor, compared to using the traditional technique of air sparging.
A method and apparatus for establishing, maintaining and enhancing microorganisms utilized to remediate groundwater or soils contamination through the injection of nutrients and gases, using a cylindrical head with radial apertures and a pointed lower end adapted to penetrate the soil, and through which a fluid can be delivered, is disclosed in Albergo et al. U.S. Pat. No. 5,133,625. The fluid, which may be a viable microorganism culture containing nutrients, or may be a gas which permits or enhances the growth of ambient microorganisms, is introduced into a subsurface location under pressure through the apertures in the cylindrical head. The pressure is provided by a pump or other means, and is adjustable. However, Albergo et al. does not suggest the use of dry media in accordance with the particular dictates of the present invention. In Billings et al. U.S. Pat. No. 5,277,518 it is suggested that an oxygen-containing gas can be used to provide microorganisms and nutrients to the subsurface, and that injection wells can be connected to an air compressor for this purpose. In Payne et al. U.S. Pat. No. 4,945,988, a sparging process and apparatus is modified by placing an oxygen separator along conduit lines leading to an aquifer downstream of an air pump, which permits the delivery of air which is substantially oxygen free to the aquifer, or is oxygen enriched to the vadose zone, thereby preventing growth of aerobic bacteria in the aquifer, while stimulating such growth in the vadose zone. In neither of these disclosures, however, is there any suggestion of pneumatic injection of dry media and the formation of discrete granular lenses in the formation, which are continuous between the injection point and adjacent vent wells in the formation, as is provided by the present invention.
Another approach taken in the art to remediating contaminated soil formations in certain situations, especially where radioactive waste contamination is involved, is to isolate the contaminated waste site by placing around it a vitrified underground structure. This is accomplished by solidification of soil by in situ melting and vitrification using heat generated in the soil itself between spaced electrodes. An improvement in this process is described in Murphy et al. U.S. Pat. No. 5,114,277, which places an initially electrically conductive material at the desired location and depth to start up vitrification underground and cause the melt to have a substantially planar shape. However, unlike the methods utilized in the present invention, the initially electrically conductive material is transported as a slurry or in a solution through horizontal boreholes, or by means of conventional hydraulic subsurface fracturing technology well known in the drilling arts.
Paramount among the limitations of the existing and emerging treatment technologies applicable to the vadose zone is the permeability of the soil formation being treated. The efficiency of in situ treatment processes all decrease as the soil permeability decreases. For soils with low permeabilities the existing processes are largely ineffective. Low soil permeability may be caused by a number of factors, including high clay content, high soil density and high fluid viscosity. An advance in this area was made by Schuring et al., U.S. Pat. No. 5,032,042, with the discovery that pneumatically fracturing of the contaminated soil formation leads to a significant improvement in the results obtained with a variety of in situ decontamination methods. However, there is no suggestion by Schuring et al. of pneumatic injection of dry media and of the techniques required for doing so, or of the manner in which a variety of such dry media may be used to enhance various remediation technologies.
The method described by Schuring et al. for eliminating subsurface contaminants from soil includes the steps of a) pneumatically fracturing the soil, including the steps of i) inserting a tubular probe partially into a well in the soil such that at least one orifice of a nozzle fluidly connected with the tubular probe is positioned at a predetermined height; ii) providing a sealed area in the well on opposite sides of the orifice; and iii) supplying a pressurized gas to the tubular probe which travels through the orifice into the soil to produce a fractured soil formation; and b) transforming the contaminants into a different state to decontaminate the soil, after creation of the fractured soil formation. There is also described therein in general terms the use of such pneumatic fracturing to provide nutrient seeding, although this is not defined, and there is no disclosure of specific devices and methods which might be used to accomplish this general objective.
For the present invention, on the other hand, there is provided herein ample description of such features as novel directional and plate-type nozzles which are able to focus high flow velocities into the soil formation, creating planar voids of substantially 360.degree. circumference, or of sectional arcs thereof, by a rapid pneumatic intrusion and cutting action, whereafter dry media in a dry carrier gas are injected rapidly to fill the formation, in accordance with the guidelines provided herein. The present invention can be used in a number of ways to obtain increased rates of contaminant reduction, removal, or isolation which are dramatic.